Liquid volume monitor for pressurized tanks

ABSTRACT

A monitor for remote reading of liquid volumes in pressurized tanks utilizes a hollow buoyant displacement probe extending downwardly from a load cell carried adjacent a top port in the tank. The load cell is suspended from a flange cover acting as a closure for the tank port so that the load cell is freely pivotable on perpendicular horizontal axes, insuring that it is always oriented horizontally. A temperature-reading string is positioned in the open hollow center of the probe and has three spaced temperature sensing units along its length. Force data from the load cell, giving the apparent weight of the probe immersed in the liquid tank contents, and temperature data, from which volumetric data may be calculated, are fed to an external battery-operated microprocessor which periodically radios its data to a central computer which may be polled by phone lines from a remote monitoring station.

TECHNICAL FIELD OF THE INVENTION

This invention relates to apparatus mountable to pressurized liquidhydrocarbon storage tanks for remote monitoring of the volume ofcontents therein, and more particularly to improvements in monitorswhich function by suspending a buoyant displacement probe in liquid tankcontents from a load cell to measure the apparent weight of the probe.

BACKGROUND OF THE INVENTION

Storage tanks for liquefied hydrocarbon products such as butane andpropane present special problems for the safe and ready monitoring oftank volume levels. This is particularly so where the tanks are locatedin remote or relatively inaccessible locations, making tank inspectionsinefficient and inconvenient.

LP gas storage tanks are classified as explosion hazards by the NationalFire Protection Association, requiring special care in the design andinstallation of any ancillary equipment. The LP Gas Code(NFPA 58)defines the area within 5 feet of any tank, fill opening or point whereLP gas is dispensed, loaded, vented or the like as a Class I, Division1, Group D hazard.

Despite the daunting nature of the problems involved in safelyinstalling a volume monitor for such a tank, it would be desirable toprovide for the remote reading of volume levels so that fuel suppliescan be maintained in adequate amounts without the necessity of sitetrips to ascertain the amount of fuel on hand.

It is known in the measuring art to determine the weight of liquidcontents in a tank by suspending a buoyant probe in the liquid from aload cell to measure the apparent weight of the probe. By well-knowncalculational formulas, such measurement yields the weight of tankcontents. Examples of such systems in the prior patent art may be foundin U.S. Pat. Nos. 5,614,672, 5,157,968, 5,132,923 and 4,244,218.

U.S. Pat. No. 5,157,968 discloses a buoyant displacement probe mountedthrough a top tank port via a load cell for the determination of theliquid tank content weight. It also provides a second buoyant probemounted via a load cell through a second tank port, so that the specificgravity of the liquid may be calculated from the second load cellreading and the content weight thus be converted to a volume. U.S. Pat.No. 5,614,672 likewise determines content weight by a load cell-mountedbuoyant displacement probe. It, however, relies for volume determinationon an assumption that the specific gravity of the liquid in the tank isa constant, known value.

Despite these efforts to provide tank monitors employing buoyantdisplacement probes for monitoring tank contents, there remains a lackof suitable devices employing this principle for safe and readyinstallation on remote hazardous storage tanks. This invention isdirected to apparatus which will satisfy this need by providing anaccurate and reliable monitoring system which can be safely installedfor monitoring of the liquid contents of hazardous storage tanks.

SUMMARY OF THE INVENTION

The object of this invention is to provide a volume monitoring apparatuswhich may be readily and safely installed on both new and existing tanksused to store liquid propane, liquid butane or like hazardous liquids.An important consideration in this invention is to maintain a highdegree of accuracy in the volume determination.

In accordance with the invention, there is provided a device formeasuring the liquid volume in a tank, including a load cell positionedadjacent a port in the top wall of the tank. A downwardly extendingdisplacement probe is supported by the load cell, and has sufficientlength so that its lower end is adjacent to, but not touching, the tankbottom. A suspension mechanism for the load cell includes a universaljoint means allowing the load cell to be oriented horizontally withoutregard to the orientation of the tank and port. The load cell monitorsthe apparent weight of the probe, and thus the weight of the liquidcontents in the tank. An elongate temperature probe is mounted in thesame port and has at least one temperature sensor for measuring thetemperature of the liquid in the tank. This temperature reading may thenbe used with the known specific gravity-temperature curve of the storedliquid to utilize the specific gravity of the stored liquid at themeasured temperature to calculate the volume of the liquid content.

In a specific embodiment of the invention, the displacement probe is anelongate hollow member. The temperature probe is a string extendingthrough the open center of the displacement probe, the string carrying aplurality of spaced thermometric units for measuring the temperature ofthe tank liquid at spaced levels of the tank.

The preferred form of the invention includes a riser pipe secured in thetank port, a mounting flange threaded on the riser pipe and a flange capsecured to the flange to close the port, the flange cap supporting ahanger bracket from which the load cell and displacement probe aresuspended by way of a universal joint assembly which insures that theload cell, in the form of a planar beam sensor is maintained in truehorizontal orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther advantages thereof, reference is now made to the followingDescription of the Preferred Embodiments taken in conjunction with theaccompanying Drawings in which:

FIG. 1 is a plan view of a monitor constructed in accordance with thisinvention mounted on a pressurized tank;

FIG. 2 is a perspective view of the hanger bracket, universal jointassembly and load cell of the device of FIG. 1;

FIG. 3 is a plan view of the portion of the apparatus shown in FIG. 2;

FIG. 4 is a plan view of the displacement probe and temperature probe ofthe device of FIG. 1, taken at right angles to the depiction of theprobe in FIG. 1;

FIG. 5 is a horizontal cross-section taken along line 5—5 IN FIG. 4; and

FIG. 6 is a schematic illustration of a monitoring system utilizing theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in the drawings, a storage tank 10 for liquefied propane,butane or similar hazardous liquid is provided with a monitoringapparatus constructed in accordance with the invention by installationthrough a single top port 12. Existing tanks are conventionally providedwith such an inspection port having a two inch diameter, and thepreferred form of apparatus of this invention may be readily and safelyinstalled in such a pre-existing port.

A threaded riser pipe 14 is secured and sealed in port 12, and extendsabove the port a few inches. Typically, pipe 14 may be about six inchesin length. A heavy-duty flange 16 is threaded and sealed to riser 14.Flange may be provided with a circular array of eight bolt holes. Aflange gasket and flange cover 18 having conventional pressure-proofelectrical cable pass-through is secured to flange 16 by conventionalmeans such as bolts 20 to close the port 12 in sealed, pressure-prooffashion.

A hanger bracket 30 is provided for suspending the in-tank elements ofthe apparatus. Hanger bracket 30 has an upper flange 32, a lower flange34, and a vertically extending web 36 connecting the flanges 32 and 34.A circuit board 38 is mounted on web 36. The details of construction ofboard 38 are conventional. Board 38 is provided with a plug 37 forconnecting to the cabling of the pass-through flange cover 20 tocommunicate the data received by board 38. Web 36 also carries atemperature sensor 41 for measuring the air temperature in the upperportion of the tank, connected to circuit board 38. This permitstemperature compensation of data from the load cell described below. Anaperture 40 is provided in upper flange 32 for alignment with thepressure measuring port of the flange cover 18 for measuring tankpressures. Upper flange 32 also has a pair of mounting holes 42 forbolting bracket 30 securely to flange cover 18.

A universal joint assembly 43 is suspended below hanger bracket 30. Theassembly 43 may be any suitable commercially available universal jointassembly, such as Par Number 64565K1 from McMaster-Carr Supply Company,depicted here. The upper body 44 of assembly 43 is secured to the lowerface of flange 34 by bolt 46. A pair of spaced legs 48 extend downwardlyfrom upper body 44, and carry an upper horizontal pivot pin 50. Thelower body 52 of assembly 43 has upwardly extending spaced legs 54 whichcarry a lower horizontal pivot pin 56. Pins 50 and 56 are oriented sothat the vertical planes through their axes are mutually perpendicular.Each of the pins 50 and 56 extend through a pivoting central body 58 ofthe universal joint assembly 43, positioned between legs 48 and 54. Thisarrangement permits lower body 52 to hang vertically plumb from pin 56,even if the lower flange 34 of hanger bracket 30 is not orientedhorizontally because of a tilt in the tank 10, the tank port 12, or forany other reason.

An upper load cell-mounting u-bracket 60 is secured to the universaljoint assembly 43 at its lower body 52 by means of a clevis pin 62secured by a cotter pin 64. U-bracket 60 has a horizontal leg 66extending therefrom. One end of a load cell 70 in the form of a planarbeam sensor is secured to leg 66 by means of a first compression[[c]]clamp 72. The opposite end of load cell 70 is secured to leg 76 of loweru-bracket 78 by second compression clamp 79. Thus, downward force onlower u-bracket 78 will produce an electrical signal from load cell 70which measures the magnitude of the force. The wiring harness 73 of loadcell 70 is connected to circuit board 38.

The effect of universal joint assembly 43 is to insure that planar beamsensor 70 is oriented horizontally. This eliminates the need formeasurement and correction for any variation of the load cell 70 fromthe horizontal. Were the cell 70 permitted to be oriented out ofhorizontal, its measurements of force would be reduced by the sine ofthe angle of deviation. Universal joint assembly 43 eliminates thissource of error, and the necessity of compensation.

Lower u-bracket 78 is provided with a clevis pin 82 secured by cotterpin 84 for mounting a buoyant displacement probe 90. Probe 90 may be ahollow tubular aluminum extrusion, and includes a vertically extendingcentral passage 92, as well as vertically extending side chambers 94provided to lighten probe 90 and increase its buoyancy. Annular covers96 are secured to each end of probe 90 to close chambers 94 whileleaving central passage 92 open to the liquid contents of tank 10. Amounting neck 100 extends from the upper end of probe 90, and isprovided with a through-hole 102 for receiving the clevis pin 82 tosuspend probe 90 from lower u-bracket 78. A second through-hole 104 isprovided in neck 100, so that a screwdriver or the like may be placedtherethrough to support probe 90 on the riser pipe 14 duringinstallation, while the installer makes the wiring connections tocircuit board 38.

Probe 90 houses a flexible temperature probe string 110 which passesdownwardly through open central passage 92. A plurality of temperaturesensors 112 are spaced along temperature probe 110 for measuring thetemperature of the liquid contents at spaced levels. In the preferredembodiment, the sensors 112 are spaced so that they are suspended at the5%, 35% and 65% of tank height levels within the tank. Each sensor 112communicates separately with a signal connector 114 located at the upperend of probe 110. Connector 114 plugs into circuit board 38 attemperature plug-in 39. This plug-in connection is adequate to supportthe temperature probe string 100, because of its light weight.

The data conveyed from the load cell 70 to circuit board 38 throughwiring harness 73, and the temperature data coming to the board throughconnector 114, is communicated externally of the tank to amicroprocessor housed in control box 120 secured atop flange. Themicroprocessor calculates the volume of contents in the tank from: (1)the apparent probe weight data from load cell 70, compensated for airtemperature surrounding load cell 70 as measured by temperature sensor41; (2) the liquid temperature data from probe 110; and (3) the specificgravity curve for the stored liquid. Control box 120 also houses a radiofrequency transmitter/receiver which can transmit the data to a mastercomputer. This eliminates the need for a power hook-up within thehazardous area of the tank, as the microprocessor and radio may beconveniently operated on safe battery power.

A suitable arrangement of the monitoring station is depicted in FIG. 5.A plurality of tanks with monitors installed as described abovecommunicate by radio to a master computer housed under roof at a nearbylocation outside the hazardous zone. The master computer can be polledperiodically by telephone from a remote monitoring station located manymiles away. Of course, when remote downloading is not required, as wherean on-site manned facility exists, the data can be accessed directly atthe master computer. Indeed, the master computer may be a pc used foroffice or other functions.

Whereas the present invention has been described with respect tospecific embodiments thereof, it will be understood that various changesand modifications will be suggested to one skilled in the art and it isintended to encompass such changes and modifications as fall within thescope of the appended claims.

We claim:
 1. A device for measuring the liquid volume in a tankcomprising: (a) a load cell positioned adjacent a port in the top wallof the tank; (b) a displacement probe supported by the load cell, andextending down into the tank, the displacement probe having a lower endnear the tank bottom, whereby the load measured by the load cell givesthe apparent weight of the probe when immersed in the liquid contents ofthe tank; and (c) a suspension mechanism mounted at the tank port forsupporting the load cell and the probe, including universal joint meansto orient the load cell horizontally without regard to the orientationof the tank and the tank port.
 2. The device of claim 1, wherein thesuspension mechanism includes a hanger bracket secured in fixedrelationship to the tank, supporting the load cell from above the loadcell, and the universal joint means comprises a first pivotal mountbetween the hanger bracket and load cell freely pivotable on a firsthorizontal axis and a second pivotal mount between the hanger bracketand load cell freely pivotable on a second horizontal axis perpendicularto said first horizontal axis.
 3. The device of claim 2, furthercomprising: (a) a riser pipe secured in the tank port and extendingupwardly therefrom; (b) a mounting flange threaded onto the riser pipe;(c) a flange cap secured to the flange, to which the hanger bracket isfastened.
 4. An apparatus for measuring the volume of liquid in a tankhaving a top port, comprising: (a) a riser pipe secured in the port andextending upwardly therefrom; (b) a flange mounted on the riser pipe;(c) a flange cover secured to the flange; (d) a hanger bracket carriedby the flange cover; (d) a load cell suspended from the hanger bracket;(e) a hollow liquid displacement probe suspended from the load cell, andextending to a lower end adjacent to, but clear from, the tank bottom;and (f) a spaced array of temperature sensors extending downwardlythrough the hollow open center of the displacement probe.
 5. A devicefor measuring the liquid volume in a tank comprising: (a) a load cellpositioned adjacent a port in the top wall of the tank; (b) adisplacement probe supported by the load cell, and extending down intothe tank, the displacement probe having a lower end near the tankbottom, whereby the load measured by the load cell gives the apparentweight of the probe when immersed in the liquid contents of the tank;(c) a suspension mechanism mounted at the tank port for supporting theload cell and the probe, including universal joint means to orient theload cell horizontally without regard to the orientation of the tank andthe tank port; and (d) means mounted adjacent said tank port formeasuring the temperature of the liquid contents of the tank.
 6. Thedevice of claim 5, wherein the displacement probe has a verticallyextending hollow chamber open to the liquid contents of the tank, andsupports a temperature measuring string extending downwardly throughsaid hollow chamber, said string having a spaced array of temperaturesensors positioned therealong.
 7. The device of claim 5, wherein thesuspension mechanism includes a hanger bracket secured in fixedrelationship to the tank, supporting the load cell from above the loadcell, and the universal joint means comprises a first pivotal mountbetween the hanger bracket and load cell freely pivotable on a firsthorizontal axis and a second pivotal mount between the hanger bracketand load cell freely pivotable on a second horizontal axis perpendicularto said first horizontal axis.
 8. The device of claim 7, furthercomprising: (a) a riser pipe secured in the tank port and extendingupwardly therefrom; (b) a mounting flange threaded onto the riser pipe;(c) a flange cap secured to the flange, to which the hanger bracket isfastened.